1. Field of the Invention
This invention relates generally to lithium ion batteries, and more specifically to a polystyrene-block-polyethylene-block-polystyrene copolymer which is blended with a homopolyer to from a film, the homopolymer then removed using an organic solvent to thus create a nonoporous film where the holes lined with the homopolymer, the film useful as a battery separator.
2. Brief Description of the Related Art
Nanoporous separators used in lithium ion technology are of considerable current interest in spite of the fact that they are an inactive component of the battery. In most cases, the separators are composed of inert semi-crystalline polyolefins such as polyethylene and polypropylene. A liquid electrolyte contained in the pores is responsible for ion transport in the battery. Although these polyolefin materials only cost about 1.30 $/kg, the cost of a typical battery separator is in the vicinity of 120-240 $/kg[1]. This large increase in price is mainly due to the complex and carefully controlled processing steps used to generate the porous structure within the separator.
One of the most widely used processes presently used in the manufacture of battery separators is the “wet process” wherein a membrane comprising a phase separated mixture of an amorphous and a semi-crystalline polyolefin is immersed in a solvent and holes are created by dissolving out the amorphous polyolefin[2]. Semi-crystalline polyolefins such as polyethylene are only soluble in solvents at elevated temperatures in the vicinity of the melting point of the crystals, which is in the vicinity of 120° C. for polyethylene. Thus the dissolution step can readily be conducted at a convenient temperature (e.g. room temperature).
Uniformity of the resulting pore structure is crucial for advanced lithium battery performance as non-uniformity of the pores will lead to non-uniform current distribution during battery operation. Defects in the separator can lead to catastrophic failure of batteries. Most polyolefins are immiscible in each other[3-7]. The extent of phase separation in the membrane prior to the pore formation step is determined entirely by non-equilibrium effects. Small changes in the processing conditions can lead to large changes in the phase separated morphology which, in turn, affects pore structure. Since driving forces for phase separation in polymers depends crucially on the molecular weight of the components [Flory and Huggins], small changes in the molecular weight distributions of the amorphous and semi-crystalline components can also result in alterations of the pore structure.
The wet process for the production of battery separators produces separators that are quite different from separators made from the dry process: specifically, the holes produced by the wet method are only minimally orientated[2]. Typically, in the wet method, polyolefin resins are mixed with paraffin oils and other additives, extruded into films, and washed with a volatile solvent to remove the paraffin oils[2, 8, 9]. The holes are produced by the extraction of the oil, and the films are stretched either before or after extraction to increase porosity, resulting in minimally oriented pore structures[8]. In contrast, the holes produced in the dry process are made entirely from stretching the films, which leads to uniaxially oriented films[2]. The uniaxially oriented films produced from the dry method only have a high tensile strength in one direction, whereas the films produced using the wet method typically have high tensile strength in all directions. The balance of tensile strength in not necessarily advantageous as tensile strength is primarily important for roll processing of polymer films, in which the polymers are only pulled along one axis[8]. The tortuosity of separators made from dry processes is also significantly lower than the tortuosity of separators produced using wet processes: separators produced from the dry process have straight, open pore structures suitable for high power density applications, while separators produced the wet process have more porous, tortuous structures more suitable for long battery life applications[2].